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Heat waves, can be exacerbated by the co-occurrence of daytime heat waves and nighttime heat waves. Over China, the Yangtze–Huaihe River basin (YHRB) is the core region of the occurrence of such “compound heat waves”, which exert profound impacts on the society and ecosystems. However, the physical mechanisms responsible for the variability of the YHRB compound heat waves remain unclear. In this study, the interannual variability of YHRB compound heat waves in peak summer (July–August) and its possible causes are investigated based on station observations across China and global reanalysis datasets. A strong link is found between the previous winter Arctic Oscillation (AO) and these peak-summer compound heat waves. During a negative AO in winter, an anomalous tripolar pattern of sea surface temperature (SST) in the North Atlantic is induced by the AO-related atmospheric circulation. Such tripolar SST pattern can persist until the following summer. By that time, oceanic forcing dominates, and positive SST anomalies over the tropical North Atlantic can excite a Rossby wave train propagating eastward from the southwest coast of North America to East Asia. This results in a northwest-southeast tilted high pressure system over the YHRB, favoring the occurrence of peak-summer compound heat waves. Such a delayed influence of the winter AO, together with the North Atlantic capacitor effect, is clearly seen in a case study of the YHBR compound heat waves in 2010. The proposed mechanism is further verified based on numerical experiments with an atmospheric general circulation model.

This study reveals a close relation between autumn Arctic sea ice change (SIC) in the Laptev Sea-eastern Siberian Sea-Beaufort Sea and subsequent spring Eurasian surface air temperature (SAT) variation. Specifically, more (less) SIC over the above regions in early autumn generally correspond to SAT warming (cooling) over the mid-high latitudes of Eurasia during subsequent spring. Early autumn Arctic SIC affects spring Eurasian SAT via modulating spring Arctic Oscillation (AO) associated atmospheric changes. The meridional temperature gradient over the mid-high latitudes decreases following the Arctic sea ice loss. This results in deceleration of prevailing westerly winds over the mid-latitudes of the troposphere, which leads to increase in the upward propagation of planetary waves and associated Eliassen-Palm flux convergence in the stratosphere over the mid-high latitudes. Thereby, westerly winds in the stratosphere are reduced and the polar vortex is weakened. Through the wave-mean flow interaction and downward propagation of zonal wind anomalies, a negative spring AO pattern is formed in the troposphere, which favors SAT cooling over Eurasia. The observed autumn Arctic SIC-spring Eurasian SAT connection is reproduced in the historical simulation (1850–2005) of the flexible global ocean-atmosphere-land system model, spectral version 2 (FGOALS-s2). The FGOALS-s2 also simulates the close connection between autumn SIC and subsequent spring AO. Further analysis suggests that the prediction skill of the spring Eurasian SAT was enhanced when taking the autumn Arctic SIC signal into account.

This study evaluates the ability of 35 climate models, which participate in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical climate simulations, in reproducing the connection between boreal spring Arctic Oscillation (AO) and its following winter El Niño–Southern Oscillation (ENSO). The spring AO–winter ENSO correlations range from − 0.41 to 0.44 among the 35 models for the period of 1958–2005. Ensemble means of the models with positive and negative AO–ENSO correlations both show strong spring sea surface temperature (SST) cooling in the subtropical North Pacific during a positive phase of spring AO, which is conducive to occurrence of a La Niña event in the following winter. However, the models with positive AO–ENSO relations produce a pronounced spring cyclonic anomaly over the subtropical northwestern Pacific and westerly anomalies over the tropical western Pacific (TWP). These westerly wind anomalies bring SST warming and positive precipitation anomalies in the tropical central-eastern Pacific (TCEP) during the following summer, which would maintain and develop into the following winter that support an El Niño-like pattern in the TCEP via a positive air-sea feedback mechanism. By contrast, the models with negative AO–ENSO connections fail to reproduce the spring AO-related cyclonic anomaly over the subtropical northwestern Pacific and westerly wind anomalies in the TWP. Thus, these models would produce a La Niña-like pattern in the subsequent winter. Difference in the spring AO-associated atmospheric anomalies over the subtropical North Pacific among the CMIP5 models may be attributed to biases of the models in simulating the spring climatological storm track.

The combined effect of the Arctic Oscillation (AO) and Western Pacific (WP) teleconnection pattern on the temperature variation during the winter in the northern hemisphere and East Asia over the last 56 years (1958/1959–2013/2014) was investigated using NCEP/NCAR reanalysis data. The study results revealed that the effect of the AO on winter temperature in East Asia could be changed depending on the phase of the WP pattern in the North Pacific. The negative relationship between the temperature of East Asia (25–45°N, 110–145°E) and the AO increased when the AO and WP were in-phase with each other. Hence, when winter negative (positive) AO was accompanied by negative (positive) WP, negative (positive) temperature anomalies were dominant across the entire East Asia region. Conversely, when the AO and WP were out-of-phase, the winter temperature anomaly in East Asia did not show distinct changes. Furthermore, from the perspective of stationary planetary waves, the zonal wavenumber-2 patterns of sea level pressure and geopotential height at 500 hPa related to the East Asian winter monsoon (EAWM) circulation strengthened when the AO and WP were in-phase but were not significant for the out-of-phase condition. An index considering the effect of both AO and WP on East Asia winter temperature was proposed. The correlation between the index and the East Asia winter temperature was statistically significant at the 99 % confidence level. The index was correlated with synoptic characteristics of the EAWM, including the Siberian High, East Asian trough, East Asian jet stream and surface air temperature.

Previous studies indicated that the spring Arctic Oscillation (AO) is an important extratropical forcing of the occurrence of El Niño-Southern Oscillation (ENSO) in the following winter. This study reveals an interdecadal weakening in the spring AO-winter ENSO connection around the early-1990s and investigates the reason of this change. Before the early-1990s, in association with the positive phase of the spring AO, an anomalous anticyclone appears over the mid-latitude North Pacific, accompanied by an anomalous cyclone over the subtropical North Pacific that is supported by the wave-mean flow interaction. Correspondingly, the sea surface temperature (SST) warming and its associated positive precipitation anomalies develop over the subtropical North Pacific, which play a crucial role in forming and maintaining the westerly wind anomalies over the tropical western Pacific (TWP) from spring to the following summer. The TWP westerly wind anomalies would induce the El Niño event in the following winter. After the early-1990s, by contrast, the Pacific component of the circulation anomalies related to the spring AO is weak and shifts northward. This is followed by weak anomalies of SST and precipitation over the subtropical North Pacific, as well as weak TWP westerly wind anomalies from spring to the following summer. Hence, the spring AO-winter ENSO connection is weak after the early-1990s. The change of the spring AO associated circulation anomalies over the North Pacific around the early-1990s tends to be related to the interdecadal change in the intensity of the Aleutian Low.

Utilizing a recent observational dataset of particulate matter with diameters less than 2.5 µm (PM2.5) in North China, this study reveals a distinct seesaw feature of abnormally high and low PM2.5 concentrations in the adjacent two months of December 2015 and January 2016, accompanied by distinct meteorological modulations. The seesaw pattern is postulated to be linked to a super El Niño and the Arctic Oscillation (AO). During the mature phase of El Niño in December 2015, the weakened East Asian winter monsoon (EAWM) and the associated low-level southerly wind anomaly reduced planetary boundary layer (PBL) height, favoring strong haze formation. This circulation pattern was completely reversed in the following month, in part due to a sudden phase change of the AO from positive to negative and the beginning of a decay of the El Niño, which enhanced the southward shift of the upper tropospheric jet from December to January relative to climatology, leading to an enhanced EAWM and substantially lower haze formation. This sub-seasonal change in circulation is also robustly found in 1982–1983 and 1997–1998, implicative of a general physical mechanism dynamically linked to El Niño and the AO. Numerical experiments using the Weather Research and Forecasting (WRF) Community Multiscale Air Quality (CMAQ) model were used to test the modulation of the meteorological conditions on haze formation. With the same emission, simulations for three super El Niño periods (1983, 1997 and 2015) robustly show higher PM2.5  concentrations under the mature phase of the super El Niño, but substantially lower PM2.5 concentrations during the decay phase of El Niño (and the sudden AO phase change), further verifying the modulation effect of the sub-seasonal circulation anomaly on PM2.5 concentrations in North China.

The connection between November Arctic Oscillation (AO) and the following spring-summer tropical Pacific sea surface temperature (SST) experienced a pronounced enhancement around the late-1970s. The connection was weak before, but strong after the late-1970s. The present study investigates the plausible reasons for the connection change. After the late-1970s, significant anomalous cyclone appears over subtropical North Pacific and pronounced anomalous westerly winds occur over tropical western Pacific in the positive phase of November AO. As such, November AO could exert a significant influence on the subsequent spring-summer tropical Pacific SST variation. By contrast, before the late-1970s, anomalous cyclone and associated anomalous westerly winds to its south are weak and shift northward. Thereby, the connection between November AO and tropical Pacific SST is weak. Further analyses suggest that the interdecadal change in the anomalous cyclone over the subtropical North Pacific may be related to the change in the November AO’s Pacific component and the strength of feedback of synoptic-scale eddy to mean flow. Specifically, after the late-1970s, November AO’s Pacific component displays a zonally elongated structure, which is favorable for the formation of easterly wind anomalies over the mid-latitude North Pacific and related wave-mean flow interaction. In addition, feedback of synoptic eddy to mean flow is enhanced after the late-1970s, which is partly related to the intensification of the North Pacific storm track. This may also contribute to enhancement of the anomalous cyclone over the subtropical Pacific.

The characteristics of the wintertime Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) and their impacts on climate variability over the Northern Hemisphere are important metrics for evaluating a climate system model. Observational analyses reveal that the horizontal and vertical structures in the AO and NAO exhibit a meridional dipole and a large-scale barotropic pattern between the Arctic and mid-latitudes. Historical model simulations from the Energy Exascale Earth System Model (E3SM-HIST) are used to identify how well it captures these major climate modes. It is found that the simulated AO and NAO modes have spatial structures similar to the observed features. In addition, the observed frequency bands in the AO and NAO-related time variability are captured well in the E3SM-HIST simulation. Associated with the positive phase in wintertime AO and NAO, zonal flow and warm advection in mid-latitude continents are enhanced, along with stronger cold flow from enhanced northerly winds over high latitudes. These features are linked to the atmospheric circulation pattern reflected by lower SLP anomalies over the Arctic and higher SLP anomalies over the mid‐latitudes. In E3SM-HIST, these spatial associations and main structural features are analogous to those in observations. In the time-height evolution related to winter AO and NAO modes, it can also be seen that the simulations reproduce the downward propagating patterns in observations. Nevertheless, the vertical structures associated with AO and NAO in E3SM-HIST exhibit substantial biases in the lower stratosphere. The cause of these stratospheric biases is investigated using the strength of climatological stratospheric polar vortex (SPV) and wave activity fluxes. The results herein suggest that E3SM-HIST has a reasonable skill in reproducing the observed characteristics related to the winter AO and NAO, although there exist systematic biases in the associated climate variability.

Arctic climate changes have profoundly influenced the polar environmental changes in recent years. The Arctic Oscillation (AO), as a key component of the Arctic climate system’s internal variability, affects the source to sink processes and interactions across the multilayer Arctic system by regulating the land, ocean, sea ice, and atmospheric processes. The East Siberian Arctic Shelf (ESAS) has experienced significant changes in the input, transport, and burial of sedimentary organic carbon (OC) due to climate warming and shifts in the AO phase in recent decades. This study analyzes grain size, total organic carbon (TOC), total nitrogen (TN), and stable carbon isotope (δ 13 C) in two sediment cores from the ESAS to reconstruct the burial record of OC over the past few decades and examine the response mechanism of sedimentary OC records to regional-scale climate forcing. The results show that the OC in the two sediment cores originates from mixed sources with a dominant terrestrial contribution. In the LV83-28 core from the Laptev Sea, the TOC and TN contents have increased at an accelerated rate since the 1990s, with a noticeable rise in the contribution of terrestrial OC. This trend is linked to an increase in terrigenous input caused by the positive AO phase. Core LV83-39 in the East Siberian Sea could have accumulated more terrestrial OC transported along the continental shelf during the positive AO. This implies that, under the interannual regulation of the AO regime, the input and cross-shelf transport of terrigenous OC in the ESAS showed consistent sedimentary responses. This finding could enhance the understanding of the burial mechanism of sedimentary OC and its environmental response to regional climate change.

In the study authors analyzed the interannual relationship between the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the tropical Indian Ocean (TIO) precipitation in boreal winter for the period 1979–2009. A significant simultaneous teleconnection between them is found. After removing the El Niño/Southern Oscillation and Indian Ocean dipole signals, the AO/NAO and the TIO precipitation (0°–10°S, 60°–80°E) yield a correlation of +0.56, which is also consistent with the AO/NAO-outgoing longwave radiation correlation of −0.61. The atmospheric and oceanic features in association with the AO/NAO-precipitation links are investigated. During positive AO/NAO winter, the Rossby wave guided by westerlies tends to trigger persistent positive geopotential heights in upper troposphere over about 20°–30°N and 55°–70°E, which is accompanied by a stronger Middle East jet stream. Meanwhile, there are anomalous downward air motions, strengthening the air pressure in mid-lower troposphere. The enhanced Arabian High brings anomalous northern winds over the northern Indian Ocean. As a result the anomalous crossing-equator air-flow enhances the intertropical convergence zone (ITCZ). On the other hand, the anomalous Ekman transport convergence by the wind stress curl over the central TIO deepens the thermocline. Both the enhanced ITCZ and the anomalous upper ocean heat content favor in situ precipitation in the central TIO. The AO/NAO-TIO precipitation co-variations in the IPCC AR4 historical climate simulation (1850–1999) of Bergen Climate Model version 2 were investigated. The Indian Ocean precipitation anomalies (particularly the convective precipitation along the ITCZ), in conjunction with the corresponding surface winds and 200 hPa anticyclonic atmospheric circulation and upper ocean heat contents were well reproduced in simulation. The similarity between the observation and simulation support the physical robustness of the AO/NAO-TIO precipitation links.